In general, gtkmm-style projects use Doxygen, which reads specially formatted C++ comments and generates HTML documentation. You may write these doxygen comments directly in the header files.
You might wish to reuse documentation that exists for the C library that you are wrapping. GTK-style C libraries typically use gtk-doc and therefore have source code comments formatted for gtk-doc and some extra documentation in .tmpl files. The docextract_to_xml.py script, from pygtk's codegen directory, can read these files and generate an .xml file that gmmproc can use to generate doxygen comments. gmmproc will even try to transform the documentation to make it more appropriate for a C++ API.
For instance,
./docextract_to_xml.py -s /gnome/head/cvs/gtk+/gtk/ -s /gnome/head/cvs/gtk+/docs/reference/gtk/tmpl/ > gtk_docs.xml
Because this automatic transformation is not always appropriate, you might
want to provide hand-written text for a particular method. You can do this
by copying the XML node for the function from your
something_docs.xml
file to the
something_docs_override.xml
file and changing the
contents.